1. Field of the Invention
This invention relates generally to the field of amplifier circuits and, more particularly, to a capacitively-coupled, wide bandwidth amplifier.
2. Related Art
A non-contact or capacitively-coupled test probe allows the probing of traces and other conductors on a circuit board without physical contact. Probing without physical contact allows signals to be picked-up through solder resist or conformal coating and reduces the probability of physical damage to the conductor being probed. Commonly owned U.S. Pat. No. 5,274,336 to Crook et al. (hereinafter the '336 patent), the full text of which is incorporated herein by reference as if reproduced in full below, discloses a capacitively-coupled test probe for non-contact acquisition of both analog and digital signals.
The probe disclosed in the '336 patent includes a probe body, a probe tip and an amplifier circuit within the probe body. The probe tip has ultra fine dimensions (e.g., a diameter on the order of 10 mils or less). This ultra fine probe tip can be used to probe the ultra fine trace geometries used on modern printed circuit boards and minimizes the capacitive loading on the circuit being probed.
A problem with capacitively-coupled probes such as that described in the '336 patent is that it is often difficult to produce sensed signals having usable amplitudes. This occurs because a capacitively-sensed signal will be attenuated by the proportion of the coupling capacitance (between the probe tip and the conductor being probed) to the sum of the input capacitance of the probe and the coupling capacitance. The coupling capacitance of the probe is proportional to the surface area of the active region of the probe tip (which acts as a capacitor plate) held in close proximity to the conductor being probed. Thus, for a fixed input capacitance, the smaller the size of the probe tip, the weaker will be the capacitively-sensed signal.
If the probe size is very small, then the signal may be attenuated beyond that which can be successfully discriminated from the electrical noise of the amplifier circuit. For example, an ultra fine probe tip as taught by the '336 patent may exhibit a coupling capacitance on the order of 10 fF (femtoFarads). The input capacitance of the probe is on the order of 1.0 pF (picoFarads). This will result in a factor of 100 (10 fF/1 pF) attenuation of the input signal.
Another problem which results from the signal being sensed through a coupling capacitance is that the transfer function of the amplifier circuit will exhibit a zero at zero hertz and a pole at some (relatively low) frequency f.sub.1. Thus, input signals will be differentiated at frequencies below f.sub.1. The location (f.sub.1) of the pole is dictated by the input capacitance and the input resistance of the probe.
In addition, the amplifier circuit of the probe will exhibit a pole at some (relatively high) frequency f.sub.2. The location (f.sub.2) of the high frequency pole is dictated by parasitic impedances within active devices of the amplifier circuit.
The region between the low and high frequency poles (f.sub.1 to f.sub.2) represents the passband of the probe or, more specifically, of the amplifier. It is desirable to improve the low frequency response of the amplifier by moving f.sub.1 down in frequency. However, frequency f.sub.1 is inversely proportional to the product of the input capacitance and the input resistance of the amplifier. Thus, in order to decrease f.sub.1, the product of the input capacitance and input resistance must be made larger. If the value of input capacitance is increased, however, then the attenuation problem discussed directly above will be worsened. Moreover, the input resistor provides a bias current to a first transistor stage of the amplifier. In order to provide the required bias current, the value of the input resistor ordinarily cannot be increased beyond a certain value. In addition, increasing the input resistance will increase the Johnson noise of the amplifier circuit. As a result, the low frequency response of known capacitively-coupled amplifier circuits is often poor (i.e., having a relatively high value of f.sub.1).
It is known to bootstrap the input capacitance of a capacitively-coupled amplifier to improve the low frequency response. This will make the input capacitance appear smaller than its actual value. As a result, the passband gain and the low frequency response will be improved. These improvements, however, are attained at the cost of reduced amplifier bandwidth (i.e., the high frequency pole moves down in frequency). In addition, the bootstrapping feedback loop must have a frequency response which is active throughout the passband of the amplifier. The stabilization of such a high frequency feedback loop is difficult, and ringing, oscillation and other problems can easily result.
What is needed is an amplifier circuit for use with capacitively-coupled inputs, such as those encountered with a non-contact probe, which exhibits a low input capacitance, a high gain, a good low frequency response, and a wide signal bandwidth to overcome the limitations of known amplifier circuits.